The actor Steve Martin has found himself caught up in the biggest art forgery scandal in German history after it emerged he bought a painting "by" German-Dutch modernist Heinrich Campendonk, which turned out to be a fake.

Martin purchased the piece, Landscape With Horses, in 2004 from a Parisian gallery. He has since resold it and told the New York Times earlier this week that he is not yet certain if he is liable to compensate the buyer.

"The gallery that sold me the picture has promised to be responsible to me, if I'm responsible, but it's still unclear," Martin, who is not accused of any wrongdoing, told the Arts Beat art blog. "It wasn't clear that it was a fake until after Christie's had sold the picture – it was a long time after that, that it became known." The actor, a long-term art aficionado who has referenced his passion in films such as LA Story, added that the forgers "were quite clever in that they gave it a long provenance and they faked labels, and it came out of a collection that mingled legitimate pictures with faked pictures".

Martin's purchase – he paid $850,000 and sold the painting for a loss at $500,000 to a Swiss businesswoman in 2006 – is just one of many believed to have been faked by a German forgery gang arrested by police last year. Alleged mastermind Wolfgang Beltracchi, his wife Helene, her sister Jeanette and accomplice Otto Schulte-Kellinghaus are accused of selling 44 of the paintings over the past decade. They include fakes of early 20th-century works by Campendonk, Max Pechstein, Max Ernst and several others.

The paintings are said to have been excellent forgeries, fooling a number of experts in the art field. The forgery gang invented backstories for most of them, suggesting they were part of two collections saved during the Nazi years: one from Helene and Jeanette's grandfather, Werner Jagers, the other from Schulte-Kellinghaus's grandfather, a tailor named Knops. Several of the paintings ended up in French galleries, including Cazeau-Béraudière, where Martin made his purchase. An expert had supposedly confirmed the piece's authenticity prior to the actor handing over his money.